In radio communications systems, such as the second generation European Mobile Radio System GSM (Global System for Mobile Communications), information such as speech, picture information or other data is transmitted on electromagnetic waves via a radio interface. The radio interface relates to a connection between a base station and a large number of subscriber stations, in which case the subscriber stations may, for example, be mobile stations or fixed-position radio stations. The electromagnetic waves are, in this case, transmitted at carrier frequencies which are in a frequency band provided for the respective system. Frequencies in the frequency band around 2000 MHz have been provided for future radio communications systems, such as the UMTS (Universal Mobile Telecommunications system), or other 3rd generation systems. Two modes are envisaged for the third mobile radio generation UMTS, with one mode being referred to as FDD operation (Frequency Division Duplex), and the other mode being referred to as TDD operation (Time Division Duplex). These modes are used in different frequency bands, with both modes supporting a so-called CDMA subscriber separation method (Code Division Multiple Access).
In mobile radio systems which use subscriber separation based on a CDMA method, rapid power control is necessary in order to ensure reliable reception of all existing communication links at the location of the base station, with few disturbances due to interference from adjacent transmission channels occurring at the same time. The rapid transmission power control is required, in particular, for real time services such as speech transmission at low speeds. Rapid transmission power control based on a closed control loop and an open control loop is implemented for the FDD mode according to the prior art; for example, as in the document ARIB “Japan's Proposal for Candidate Radio Transmission Technology on IMT-2000: W-CDMA”, Jun. 1998, Japan, pages 39 to 42. The use of the open or closed control loop is, in this case, dependent on the respective type of transmission channel. The rapid closed control loop is based on so-called TPC bits (Transmitter Power Control), which are signaled periodically from the respective controlling device in the radio communications system to the other device. A base station thus controls the transmission power of the subscriber station, and vice versa.
A transmission power principle such as this is also, in principle, intended to be used for the TDD mode for harmonization between the two modes in the UMTS mobile radio system.
In order to keep the interference in the TDD mode low, and to stabilize out fading effects in the reception power level, an open power control loop is used in the uplink direction UL and a closed power control loop is used in the downlink direction DL, with these generally being referred to as power control. This results in the inner and outer control loops in the downlink direction. The outer loop measures the quality of the input signal (for example, frame error rate) and then presets a nominal value (SIR target) for the signal/interference ratio (SIR), using which the required quality can be provided. The inner loop measures the actual signal/interference ratio of the received signal, and compares this with the nominal value for the signal/interference ratio. Depending on the result, a power control command is transmitted to the opposite end, to change the transmission power level appropriately. The inner loop thus operates on a frame basis and very quickly, while the outer loop varies the nominal value only slowly. However, this type of power control is dependent on both transmission and reception taking place permanently. The following text considers, in particular, the power control process in the closed loop.
In many situations, such as when there is no data to be transmitted, such as in the case of discontinuous transmission (DTX) of radio frames, the transmission between one terminal, such as a mobile station, and a base station is, however, interrupted, at least in one direction. It is then either impossible to transmit a power control command, and/or no signal is available for reception measurements. The latter situation is important at the time at which the transmission is intended to be resumed, in which case, however, the transmission power level is then undefined, as a result of a lengthy interruption. Furthermore, in this situation without an input signal, both the inner power control loop and the outer power control loop are interrupted. The described problem occurs mainly in TDD systems, such as in UTRA-TDD, when the transmission there is interrupted at times in the event of discontinuous transmission, and the power control loop is hence also interrupted. In the case of CDMA systems using an FDD operating mode, this is generally not critical, since at least control information is still transmitted during an interruption in the data.
In the case of present-day systems, the transmission power has a safety supplement added to it after an interruption; that is to say, transmission takes place at an increased intensity. However, this produces unnecessary interference.
The present invention is, therefore, directed toward a method which allows improved transmission power control during interrupted data transmission in a radio communications system.